1. Field of the Invention
This invention relates to a group-3 nitride-based semiconductor light emitting device, in particular to a single-crystal layer used for the light emitting device and a method for manufacturing the light emitting device.
2. Description of the Related Art
Extensive research is now underway on gallium nitride (abbreviated as GaN hereinbelow) and related compounds as a material system for shortwave light emitting device, in particular shortwave semiconductor lasers. A GaN semiconductor laser device is manufactured by successively depositing semiconductor single-crystal layers such as (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a crystal substrate. Metal organic chemical vapor deposition method (abbreviated as MOCVD hereinbelow) is generally used to produce such a single-crystal layer. In this method, source gases containing trimethyl gallium (abbreviated as TMG hereinbelow) as a group-3 precursor material and ammonia (NH3) as a group-5 precursor material are introduced into a reactor to react at a temperature within the range of 900-1000xc2x0 C., thereby depositing compound crystals on the substrate. A multi-layer structure comprising various compounds can be obtained by changing the ratio of the precursors fed into the reactor to deposit many different layers on the substrate.
If the deposited single-crystal layers have many penetrating defects, the light emitting performance of the device is deteriorated substantially. Such defect is called threading dislocation, which is linearly extending defect that penetrates the crystal layer along the growth direction. Since a threading dislocation acts as a non-radiative recombination center for carriers, a semiconductor light emitting device comprising a layer with many dislocations suffers from poor luminous efficiency. The above mentioned defect is generated due to crystal misfit strain at an interface between the substrate and an overlying layer formed thereon. To reduce the influence of the misfit at the interface, attempts are made at choosing substrate materials having similar crystal structure, lattice constant, and thermal expansion coefficient to those of GaN-based crystal.
Bulk GaN crystal obviously satisfies the above requirements, however, growing single-crystal GaN with sufficient size is difficult. In addition, a conventional substrate material like GaAs is unstable under the growth temperature in MOCVD described above. Thus, sapphire is commonly used as a substrate for MOCVD growth of nitride semiconductors. Though sapphire has a substantially different lattice constant from that of GaN by about 14%, sapphire provides good stability at higher temperatures.
One approach, known as two-step-growth method, was proposed to accommodate the misfit at the interface between a sapphire substrate and a Ga-based single-crystal layer grown thereon. By using this method, a buffer layer consisting of aluminum nitride (AlN) is formed on the sapphire substrate at a lower temperature within the range of 400-600xc2x0 C., a GaN single-crystal layer is then formed over the low-temperature buffer layer. However, the above method has not been completely successful in reducing the generation of such defects that pass through the GaN single-crystal layer.
A main object of the invention is to provide a nitride semiconductor light emitting device having good luminescent characteristics.
Another object of the invention is to provide a method for manufacturing the above nitride semiconductor light emitting device whereby the generation of defects passing through a single-crystal layer formed on a substrate can be reduced.
The present invention features a nitride semiconductor light emitting device having multi-layer structure provided by depositing a group-3 nitride semiconductor on a flat substrate. The multi-layer structure comprises a first crystal layer containing substantially pyramidal crystal grains, each of which having a crystal face non-parallel to a surface of the substrate, the grains being distributed at random like islands; and a second crystal layer formed on said first crystal layer from a compound having a lattice constant different from that of the first crystal layer, the second crystal layer for transferring the surface of said first crystal into a flat surface parallel to the surface of the substrate.
The present invention further features a method for manufacturing a nitride semiconductor light emitting device having multi-layer structure, which is provided by successively depositing group-3 nitride semiconductor layers (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a flat substrate in turn. The method comprises the steps of: forming a first layer containing pyramidal nitride crystal grains being distributed at random like islands, each of the pyramidal grains has a face non-parallel to a surface of the substrate, and forming a second crystal layer over said first layer with a surface of said second layer being parallel to a surface of the substrate, said second crystal layer having a different lattice constant from that of said nitride crystal grains.
Thus, the nitride semiconductor light emitting device of the present invention has lower density of threading dislocations passing through the multi-layer structure. Accordingly, the present invention can provide a nitride semiconductor light emitting device having superior luminescent characteristics.